Melanin is known for its use as a pigment to provide a naturally appearing tan and a sunscreen to mammalian skin and hair. In recent years, increased attention has been given to the application of melanin for providing full spectrum (ultraviolet, visible and near infrared) absorption properties to protective eye wear. Ordinary eyewear lenses only cut off the ultraviolet radiation below 400 nm. It is believed that the high-energy visible (HEV), which is the radiation in the range of wavelength between 400 nm and 550 nm, causes photochemical damage to the retina. The amount and severity of the damage increases exponentially as the wavelength of the radiation decreases towards 400 nm. Melanin is effective in blocking HEV.
The application of melanin as absorbing pigment against radiation emitted by artificial and natural light sources is specifically disclosed in a series of U.S. patents: U.S. Pat. Nos. 4,698,374, 5,036,115, 5,047,447, and 5,112,883.
The use of a polarizing plate containing melanin in making sunlight protection eyewear has received much attention recently, due to the additional elimination of HEV light by melanin. U.S. patent application Ser. No. 10/207,307 describes a melanin-containing polarizing plate, in which melanin shares the same host layer as the polarization element. Usually, a polyvinyl alcohol (PVA) type polymer is used as the host film material. A hydrophilic melanin and polarization element (dichroic dyes or iodine crystal) are incorporated into the PVA film by means of absorption. In a typical process to make a high quality polarizing film such as that disclosed in U.S. Pat. No. 4,992,218, a PVA film (or partially stretched film) is soaked in an aqueous solution of dichroic dyes first. It is subsequently stretched substantially in one direction. The stretched film is then stabilized by immersing it in an aqueous solution containing a metallic ion and boric acid. The film is finally treated with heat. The absorption of melanin can happen before, during, or after the dichroic dye absorption.